The present invention relates to the field of treating aqueous effluents containing cations.
The extraction of some metals from aqueous effluents using ion exchange resins is already known. For example, French Pat. No. A-2,068,158 describes the extraction of strontium-90 from an irradiated effluent by making this effluent sufficiently alkaline to give it a pH of at least 10 and then by passing it through a carboxylic acid cation exchange resin.
U.S. Pat. No. 4,156,658 discloses the binding of radioactive ions, such as strontium-90 and cesium-137, in the soil by first injecting a chemical composition containing the radioactive ions into the soil. The chemical composition contains sodium acrylate, acrylamide and N,N'-methylene-bisacrylamide, which are capable of being polymerized to form a gelled structure. An initiator and a catalyst is then injected into the soil to accomplish the polymerization and the formation of an ion exchange gel in the soil so that the soil and the ions are physically bound by the gelled structure and the ions are chemically bound by the ion exchange properties of the gel.
U.S. Pat. No. 4,235,737 discloses the treatment of an aqueous or organic liquid containing dissolved or suspended radioactive particles and having a pH greater that 5.5. In this treatment an effective amount of an absorber consisting of an alkali metal salt of a polyacrylonitrile grafted onto starch is added to the liquid. The radioactive element that is particularly relevant to this process is iodine and the quantity of polymer employed for this treatment preferably varies between 1 and 2,000 times the weight of the radioactive liquid.
Moreover, U.S. Pat. No. 3,995,009 describes a process of ion exchange by bringing a cationic exchange resin in the ammonium form, such as, for example, an acrylic-divinylbenzene copolymer containing a carboxylic acid functional group, into contact with an aqueous solution of uranyl fluoride. U.S. Pat. No. 3,216,786 describes a process for the recovery of heavy metals from a solution having a pH of at least 4 and containing compounds in which metals are present in the form of cations. In this process, the solution is first brought into contact with a slightly acid ion exchange resin that is a cross-linked matrix to which a carboxylic acid is bound. The matrix is produced by copolymerizing from 0.5% to 50% by weight of a polyethylenically unsaturated (cross-linked) monomer and from 50 to 99.5% by weight of a monoethylenically unsaturated monomer (for example acrylic acid or methacrylic acid) in an organic solvent and by removing the occluded organic solvent from the crosslinked copolymer to obtain a spongy structure.
The ion exchange resin is regenerated by washing the resin with an acid to remove the absorbed heavy metal.
Finally, French Pat. No. A-2,553,678 described N-alkyl- or N-alkylene-substituted acrylamide or methacrylamide polymers and copolymers of the (meth)acrylamides containing up to 30% by weight of at least one ionic comonomer such as especially acrylic acid, methacrylic acid and their salts. These (co)polymers, prepared, for example, in an inverse phase suspension, have a water absorption capacity that varies according to temperature so that, once heated, they undergo a contraction even in the presence of a large excess of water to release the water once absorbed. Their water absorption capacity is not substantially affected by the presence of an inorganic salt in the water. When an aqueous solution contains a low molecular weight solute such as, for example, an inorganic salt, it can be absorbed by this polymer while it still contains the solute.
Thus, it emerges from this review of the state of the art that acrylic polymers in the form of exchange resins or in underground extraction have been proposed for treatment of aqueous effluents containing especially radioactive ions. In the case of U.S. Pat. No. 4,235,737, the gravimetric quantity of the polymer employed, which is always greater than the quantity of the liquid to be treated, restricts the application of the process to effluents of particularly high value, because of the high cost of the polymer.
An ion exchange resin is an unswelling solid compound which retains its physical structure during the ion exchange process. In contrast, acrylic polymers having a certain water absorption capacity are capable of passing from a powdery state to a gel state, i.e. of changing their physical structure.